A Salubrious Heavy Bombardment: The Case for Phosphorus

Recent evidence from geochemistry, lunar chemistry, and planetary dynamics suggests that the inner Solar System was pummeled 700-800 million years after the formation of rocky planets (Cohen et al., Science 2000). Intriguingly, the timing of this event closely corresponds with the first evidence of life 3.8 billion years ago (e.g., Mojzsis et al., Nature 1996). The late heavy bombardment event likely influenced the origin or evolution of life on the earth. Estimates of its effect range from deleterious through impact frustration (Maher and Stevenson, Nature 1988), to salubrious by delivery of organic compounds (Chyba and Sagan, Nature 1992). Meteorites like the Murchison chondrite have shown that organic compounds are delivered to the surface of the earth. However, such meteorites are rare and probably delivered less than 10 kg per km² per year during the heavy bombardment period (Pasek and Lauretta, Origins Life Evol. B. 2008). In contrast to organic delivery, I propose phosphorus delivery was an especially beneficial phenomenon during the heavy bombardment period.

Phosphorus is a key biologic element. It forms the backbone of DNA and RNA, and shuffles energy as ATP. However, phosphorus on the earth is limited to orthophosphate minerals, which tend to be poor sources of phosphorus for abiotic production of organophosphates. Comparatively, meteoritic phosphorus resides in reactive phosphide minerals like schreibersite, (Fe,Ni)₃P, which corrodes in water to form reduced P compounds, and reacts with organics forming organic P compounds (Pasek et al., Geochim. Cosmochim. Acta 2007). Post-bombardment P geochemistry might have been more favorable to the production of key precursors of life due to the large influx of meteoritic P. This hypothesis suggests that P compounds in rocks deposited soon after the heavy bombardment period may have included reduced species, providing a tracer on the biologic effects of the late heavy bombardment (Pasek, PNAS 2008).